1. Field of the Invention
The invention relates to a control apparatus for an automatic transmission mounted in a vehicle. More particularly, the invention relates to a control apparatus that controls clutch-to-clutch shifting.
2. Description of the Related Art
An automatic transmission mounted in a vehicle is formed of a combination of a torque converter to which output from an engine is input and a speed change gear mechanism driven by the output of the torque converter. The power transfer path in this speed change gear mechanism can be changed by selectively applying and releasing a plurality of frictional engagement elements such as clutches and brakes, to automatically shift the transmission to a predetermined speed according to the desire of the driver or the running state. In this type of automatic transmission, a frictional engagement element for engine braking can also be used in addition to a frictional engagement element for shifting. This frictional engagement element for engine braking usually transmits power only during driving, and activates the engine brake during low speed running when in gear stages of the first or second ranges or the like by engaging at a predetermined gear stage in those ranges.
In this kind of automatic transmission, there may be a shift by switching the frictional engagement element that is applied, in which control that engages and control that releases different frictional engagement elements are executed simultaneously (i.e., a so-called “clutch-to-clutch shift”). With this type of clutch-to-clutch shift, the timing of the engagement and release of the clutches is able to be balanced, which is a preferable shifting characteristic (e.g., the driver feels a smooth shift).
In clutch-to-clutch shifting, when the clutch to be released is released too early or when the clutch to be engaged is applied too late, a situation occurs in which neither of the clutches are sufficiently engaged, and a phenomenon in which the speed of the turbine, which is the speed on the input side of the automatic transmission, increases abruptly results (hereinafter, referred to as “abrupt turbine speed increase”). When this occurs, a smooth shift feel is no longer able to be achieved so abrupt turbine speed increase learning control is executed which delays hydraulic pressure control of the clutch to be released or speeds up hydraulic pressure control of the clutch to be engaged accordingly.
On the other hand, when the clutch to be released is released too late or when the clutch to be engaged is applied too early, a situation occurs in which frictional engagement elements of different gear stages are applied at the same time, and a phenomenon in which shift shock (so-called “tie-up shock”) results due to the difference in the rotation speeds of the frictional engagement elements. When this occurs, a smooth shift feel is no longer able to be achieved so tie-up learning control is executed which speeds up hydraulic pressure control of the clutch to be released or delays hydraulic pressure control of the clutch to be engaged accordingly.
However, the foregoing abrupt turbine speed increase learning control and the tie-up learning control control the hydraulic pressure control in opposite directions. That is, with respect to the hydraulic pressure control of the clutch to be released, the abrupt turbine speed increase learning control is performed to delay that hydraulic pressure control and the tie-up learning control is performed to speed up that hydraulic pressure control. Even after the abrupt turbine speed increase learning control has been performed, if the tie-up learning control continues, there is a possibility that the control of the hydraulic pressure circuit of the clutch to be released, which is the object of control, will begin hunting. This is illustrated in FIG. 8.
As shown in FIG. 8, it is determined whether the transmission is in the abrupt turbine speed increase state or the tie-up state. When the transmission state is on the abrupt turbine speed increase side, the abrupt turbine speed increase learning control is executed to delay the hydraulic pressure control of the clutch to be released or speed up the hydraulic pressure control of the clutch to be engaged appropriately. As a result, the transmission state will be on the tie-up side after the next clutch-to-clutch shift. At this time, the learning control performed brings the transmission state into a region of slight tie-up, which is a region preferable for shifting.
Because it is difficult to detect when the transmission is in this slight tie-up state, it may be determined that the transmission in this state is simply on the tie-up side, and the tie-up learning control may be executed as a result, thereby speeding up the hydraulic pressure control of the clutch to be released or delaying the hydraulic pressure control of the clutch to be engaged. As a result, the transmission state will be back on the abrupt turbine speed increase side after the next clutch-to-clutch shift. Repeatedly executing learning controls that lead to these kinds of opposite results, results in a control state of hunting. When hunting occurs, a smooth shift feel is unable to be achieved.